The present invention relates to sewing machines in general, and more particularly to improvements in machines wherein the workpiece must be moved back and forth (forwardly and backwards) during treatment at the thread-applying station. The invention also relates to means for transporting workpieces in sewing machines of the just outlined character.
European patent application No. 106 810 discloses a stitching machine wherein the workpiece is tensioned and is caused to move backwards (a) by a pair of rollers which clamp the workpiece downstream of the stitching station and (b) by a dancing roll or pulley which is caused to tension the workpiece upstream of the stitching station so as to take up the slack which develops during rearward transport of the workpiece. The dancing roll is set in motion by a rack and pinion drive which causes it to store a length of the workpiece and to subject such length of the workpiece to a predetermined tension. A drawback of this proposal is that the machine cannot move the workpiece back and forth with a requisite degree of accuracy because the clamping and advancing action of the aforementioned rollers varies as a function of changes in the diameter and in the mass of the supply of convoluted workpiece material which is to be treated in the machine. Moreover, the force with which the rollers draw the workpiece depends on the inertia of moving parts and on the degree of accuracy with which the dancing roller is moved by the rack and pinion drive. When the workpiece is moved backwards, it is acted upon only by the roll which is shifted by the rack and pinion drive regardless of eventual fluctuations of one or more parameters such as the quality of the workpiece, the thickness of the workpiece and others. Consequently, tension in the length of the workpiece which is adjacent to the treating station changes to thus influence the quality of the pattern which is stitched, embroidered or otherwise formed in the workpiece.
German Offenlegungsschrift No. 30 02 340 discloses a modified sewing machine wherein a first pair of clamping rollers is provided upstream and a second pair of clamping rollers is provided downstream of the treating station. A drawback of such proposal is that the inertia of the supply of convoluted workpiece material affects the forward transport of the workpiece but is not a factor when the workpiece is caused to move backwards. This prevents the machine from making patterns with a required degree of predictability and reproducibility because it is not possible to advance one and the same increment of the workpiece back into a position of accurate register with the needle or needles upon completion of one or more rearward steps.